Particles of a resin such as an acrylic resin and a styrene resin have been widely used in an application of an optical material, cosmetics, a molding resin, an additive to a coating material, or the like. In each application, a demand has been expressed for resin particles in quality having a uniform particle diameter.
As a production method for obtaining the resin particles having the uniform particle diameter, disclosure has been made as to a production method for classifying resin particles in a wide particle size distribution range as produced by suspension polymerization or spray polymerization, a production method for swelling seeds using seed particles having a controlled particle diameter in seed polymerization, thereby achieving a fixed particle size distribution. However, any of these production methods has needed a plurality of steps, resulting in cost increase and complicated work.
In order to solve these problems, study has been conducted for controlling a dispersion state or a surface state of polymerizable monomer-containing oil liquid in a reaction system in suspension polymerization to give resin particles having a uniform particle diameter. For example, a method is known in which aqueous suspension polymerization is performed in the presence of colloidal silica or alkali metal halide as a suspension stabilizer (Patent Literature 1), suspension polymerization is performed in the presence of hydrophobic inorganic oxide, a hydrophilic organic compound and water-soluble neutral salt (Patent Literature 2), suspension polymerization is performed in the presence of a suspending agent (Patent Literature 3), suspension polymerization is performed in the presence of a sparingly-soluble compound (Patent Literature 4), or the like.
However, these methods have involved problems on treatment of a reaction liquid because a polymerization system contains a metal or an inorganic substance. Moreover, an effect is not obtained unless an amount of addition of an inorganic compound such as silica satisfies a fixed level or more, and therefore silica deposited on a surface of resin particles obtained may occasionally become an obstacle in subsequent processing. In addition thereto, fine particles are generated also in these methods, and a demand has not been satisfied yet.
Moreover, resin particles having excellent dispersibility are also known in which the resin particles are formed by copolymerizing (meth)acrylate ester and a monomer having a carboxyl group, and coated with a nonionic surfactant. However, such particles are obtained through a step for additionally treating a surface of the resin particles, and a plurality of steps are needed for production (Patent Literature 5).
Therefore, desire has been expressed for further efficiently dispersing and stabilizing polymerizable monomer-containing oil droplets in suspension polymerization to give resin particles having good processability and a uniform particle diameter.